Static typing

Good support for static typing both in array libraries and array-consuming code is desirable. Therefore the exact type or set of types for each parameter, keyword and return value is specified for functions and methods - see Function and method signatures. That section specifies arrays simply as array; what that means is dealt with in this section.

Introducing type annotations in libraries became more relevant only when Python 2.7 support was dropped at the start of 2020. As a consequence, using type annotations with array libraries is largely still a work in progress. This version of the API standard does not deal with trying to type array properties like shape, dimensionality or dtype, because that’s not a solved problem in individual array libraries yet.

An array type annotation can mean either the type of one specific array object, or some superclass or typing Protocol - as long as it is consistent with the array object specified in Array object. To illustrate by example:

# `Array` is a particular class in the library
def sin(x: Array, / ...) -> Array:


# There's some base class `_BaseArray`, and there may be multiple
# array subclasses inside the library
A = TypeVar('A', bound=_BaseArray)
def sin(x: A, / ...) -> A:

should both be fine. There may be other variations possible. Also note that this standard does not require that input and output array types are the same (they’re expected to be defined in the same library though). Given that array libraries don’t have to be aware of other types of arrays defined in other libraries (see Dependencies), this should be enough for a single array library.

That said, an array-consuming library aiming to support multiple array types may need more - for example a protocol to enable structural subtyping. This API standard currently takes the position that it does not provide any reference implementation or package that can or should be relied on at runtime, hence no such protocol is defined here. This may be dealt with in a future version of this standard.